16 Dr. Karl Heumann's Contributions to 



pressure ; and he concluded that the cause of this increase was 

 to be traced to the presence of the diluting gas. He supposed 

 that there is a " momentary combustion taking place in the 

 lowest part of every flame ;" this can only be when the issuing 

 gas is mixed with a due proportion of air ; therefore Blochmann 

 supposed that the explanation of the increased distance between 

 flame and burner, which is observed to take place when coal-gas 

 is diluted with an inert gas, was to be found in the following 

 statement: — " The greatly diluted gas issuing from the burner 

 at once becomes mixed with air. In order to maintain the 

 constancy of the flame this mixture must contain a fixed quan- 

 tity of combustible gas. But that this quantity may be main- 

 tained, in the case of a diluted gas, at the same point as if the 

 diluting gas were absent, a much larger volume of the issuing 

 gas must become mixed with the air; that is, the space between 

 flame and burner must be increased." 



The following facts are, I think, opposed to Blochmann's 

 somewhat strained explanation. Where a cold object touches the 

 flame, a dividing space, similar to that noticed between flame and 

 burner, is always observed. The colder the object and the 

 more diluted the burning gas, the greater is the observed space. 

 If a flame be diluted with a considerable excess of carbon 

 dioxide, for example, a piece of thick metallic wire brought 

 into this flame causes a clear space around itself, which increases 

 in proportion to the amount of carbon dioxide present. 



This experiment is best carried out in a darkened room : it 

 is always difficult to distinguish the limits of the very slightly 

 luminous flame, even if a dark background be employed. 



These facts point to the conclusion that withdrawal of heat 

 from the flame by means of the upper part of the burner is the 

 cause of the observed vacant space, and that to the same cause 

 (withdrawal of heat) is to be assigned the extinction of the 

 flame in the neighbourhood of a cold object. The explanation 

 of the increase in the distances between flame and burner, or 

 cold object, brought about by the presence of diluting indif- 

 ferent gases, is to be found in the fact that the presence of 

 such gases lowers the flame-temperature, by causing a partition 

 of the quantity of heat needed to maintain a given quantity 

 of the coal-gas in a state of combustion throughout a' greatly 

 increased volume of gas. If the temperature of the flame be 

 already low, the further decrease occasioned by the introduc- 

 tion of a cold body, although small in actual amount, is suffi- 

 cient to cool a considerable extent of gas beneath the ignition- 

 temperature: the flame is therefore extinguished in this cold 

 space. 



If this be the true explanation of the production of the 



